Process and compositions for repulping paper

ABSTRACT

The present invention relates to a process and composition for the recovery of fibre from waste paper and paper broke which has been treated with wet-strength resins. Specifically the present invention is directed to the removal of such resins from the fibre material, so that such fibre material may be recycled to a paper web forming process, by using selected persulfates such as potassium persulfate. Optionally, the persulfate is used in combination with a surfactant. Improved repulping of the waste paper is achieved.

FIELD OF THE INVENTION

[0001] The present invention relates to a process for the recovery of fibre from waste paper and paper broke which has been treated with wet-strength resins. Specifically the present invention is directed to the removal of such resins from the fibre material so that such fibre material may be recycled to a paper web forming process.

BACKGROUND OF THE PRIOR ART

[0002] The use of paper under conditions in which the paper must be used in the wet state or to absorb moisture has become one of the major utilities of paper products. Paper normally achieves its integrity or fibre to fibre strength by bonding between the fibres, possibly by hydrogen bonding of the various molecular components of the fibre. Such fibre to fibre bonding provides adequate strength for papers used in the dry condition, such as writing or printing papers or packaging which is not subjected to outdoor atmospheric conditions.

[0003] However, with the advent of paper products such as, for example, disposable towels prepared from paper, disposable paper diapers, and, in general, the need for packaging which maintains its strength under moisture conditions such as outdoor or under outdoor atmospheric applications, it has become necessary to provide additional bonding for these paper products in order to maintain what is called wet-strength. Wet-strength is developed in paper products by various coating methods which provide continuous or discrete depositions of polymeric resins which effectively bond portions of the paper product.

[0004] Prior to the advent of wet-strength resin coated paper products, recycled paper, waste paper, clippings, and paper broke were easily recycled to the paper forming processes of paper mills by various mild digestions of the recycled paper before being returned to the paper furnish. Even the use of water soluble resins for paper coating provided no problem for those interested in recycling the various paper wastes.

[0005] However, the wet-strength resins have provided difficult problems for the reuse of waste paper, such that such waste paper in many instances is burned or landfilled rather than subject the paper to expensive or poor yield cleansing processes.

[0006] In U.S. Pat. No. 2,872,313 a process for defibering wet-strength paper broke is set forth in which oxidizing salts in a dilute aqueous solution are utilized to loosen the fibre to resin bonds. The paper broke is soaked in the oxidizing solution with preferably mild agitation. The oxidizing salt agents can be selected from sodium hypochlorite, sodium chlorite, sodium chlorate, potassium chlorate, ammonium persulfate and sodium peroxide. The solution utilized generally has a pH between 7 and 10. Elevated temperatures may be utilized in the range of 115° to 180° F.

[0007] The removal of thermosetting wet-strength resins from paper is addressed in U.S. Pat. No. 3,245,868. Inclined screw fed vessels are used to handle the pulp. The resin can be removed by either acidic solutions for formaldehyde resins or basic solutions (caustic soda) for removing polyamide resins. Temperatures as high as 149° C. are contemplated, but the patent does not direct itself to paper brightness enhancement. Additionally, the patent contemplates caustic strengths to dry fibre pulp of only 4%.

[0008] U.S. Pat. No. 3,427,217 discloses a recycle process for wet-strength resin coated broke wherein an oxidizing salt such as sodium hypochlorite is used to facilitate the pulping of the coated paper. Sodium hydroxide can be used to provide a basic solution. The wet-strength resins can include polyaminopolyamide-epichlorohydrin resins, urea-formaldehyde and melamie-formaldehyde resins, and the like.

[0009] Although the use of hypochlorite is common because of its relatively low cost, in general, the use of hypochlorite has become less favoured because of the possibility of forming chlorinated organic products in the process effluents.

[0010] U.S. Pat. No. 3,674,358 discloses a process for repulping of wet-strength resin coated paper or paperboard using a non-chlorinating oxidizing agent, such as a persulfate, in combination with a buffer to maintain the process pH at a level between 7 and 12.

[0011] U.S. Pat. No. 5,718,837 describes a process using a composition containing a mixture of a persulfate and additional, different buffering agents.

[0012] Further, U.S. Pat. No. 5,904,808 describes a further process using a persulfate and a buffer which process is conducted at a pH level of between 2.3 to 6.5. The buffering systems used in this process are buffers which will maintain these acidic pH levels.

[0013] Alternatively, U.S. Pat. No. 4,416,727 provides a process wherein the oxidizing agent is a high pressure oxygen system. This process is stated to be particularly appropriate for the removal of thermoplastic resins.

[0014] While these processes have met with some success in the repulping of paper or paperboard products, there continues to be a need to develop improved repulping processes for wet-strength resin containing products. Further, it should be noted that these processes have generally contemplated the use of hypochlorite or persulfates, and in particular, sodium or ammonium persulfate. While other persulfates have been mentioned or described, the art has typically used these two persulfates because of their higher water solubility which is felt to be advantageous in the treatment of the recycled paper, and their higher activity compared to other persulfates.

[0015] It has now been found, however, that potassium persulfate, provides improved performance over prior art systems comprising sodium or ammonium persulfate.

SUMMARY OF THE INVENTION

[0016] Accordingly, the present invention comprises a process for the recovery of fibres, particularly cellulose fibres, from waste paper or paperboard, paper trimmings and paper broke which have previously been treated with a wet-strength resin. Thus, in one aspect the present invention provides a repulping process for use on waste paper containing a wet-strength resin, comprising treating said waste paper with a mixture of an alkali hydroxide and essentially 100% potassium persulfate.

[0017] By “essentially 100% potassium persulfate” is meant a material comprised of greater than 99% potassium persulfate which contains only contaminant levels of other materials.

[0018] In an additional aspect of the present invention, it has been found that the activity of persulfates in general is improved by the addition of a surfactant to the repulping process. Accordingly, the invention also provides a repulping process for use on waste paper containing a wet-strength resin, comprising treating said waste paper with a non-chlorinated oxidizer in combination with a surfactant.

[0019] In the practise of the process of the present invention, it should be noted that the production of wet-strength resin coated papers such as diapers, towels and weather resistant packaging material constitutes a greater and greater percentage of the present day paper production. In the past, strength imparting coatings have been applied to paper products, which coatings were easily hydrolyzed or dissolved. With the improvement of wet-strength paper products wherein polymeric resins are used as binders for the paper products, a substantial loss in paper fibre is realized due to the inability to reprocess waste paper or paper broke.

[0020] Paper broke is that fraction of the paper product of a paper production which is rejected for any one of a number of reasons, such as crushed, damaged, or wrinkled paper, trim from a paper roll or other rejected paper occurring during the paper production process.

[0021] This paper broke constitutes a substantial portion of the total paper fibre processed in paper production, sometimes 20% of the paper processed. Such paper broke was easily recycled when binders were not used or the binders were easily dealt with.

[0022] However, with the use of polymeric resins as binders especially crosslinked resins, the ability to produce recyclable paper fibre from paper broke has been curtailed. The broke from wet-strength resin coated papers has in many instances been land filled or burned due to the inability to economically recycle the fibre content.

[0023] The present invention overcomes this inability and is effective on previously difficult polymeric resin coatings on paper broke.

[0024] Repulping of the waste paper, or the like, typically consists of subjecting the paper stock or paper broke to an alkaline, aqueous solution to provide a pulp slurry consisting of 2% to 25% pulp, and contacting the pulp with the oxidizer salt while maintaining the slurry at an elevated temperature. In this manner, the pulp fibres are separated from the wet-strength coating in such a manner that the brightness of the fibres is maintained. The pulp slurry is then diluted (if necessary) to a pulp concentration (also know as pulp “consistency”) of 2% to 8% by weight, and the separated fibres are screened in order to recover the individual fibres from any non-screenable materials. The screened and separated fibres can then be recycled to a paper furnish for the production of paper sheets, or other paper products.

[0025] In this manner, wet-strength paper production processes can utilize substantially more of the total fibre content of paper formed in the paper furnish by recovering the paper broke without resort to the prior art practice of burning or disposing of the valuable paper fibre content of the broke which is normally a by-product of wet-strength paper production processes.

[0026] The non-chlorinated oxidizer is preferably an alkali or alkali earth metal persulfate or ammonium persulfate. More preferably, the oxidizer is an alkali metal persulfate, and most preferably, the oxidizer is potassium persulfate.

[0027] The level of the non-chlorinated oxidizer used in the repulping process is preferably in the range of from 0.1 to 10% by weight on the weight of pulp and oxidizer. More preferably, the level of oxidizer is between 0.3 to 7.5%, and most preferably is between 0.5 to 5% by weight.

[0028] Various chemicals may be added to adjust the pH of the system. Preferably the repulping process is conduct at an alkali pH of between 8 to 12, and more preferably, between a pH of between 9 to 11. Adjustment of the pH to these pH levels can be achieved by adding various alkali materials such as alkali metal hydroxides such as sodium or potassium hydroxide, but can include various buffers such as alkali metal carbonates or bicarbonates such as sodium carbonate. The process can also be conduct under acidic conditions and for these acidic systems, acids such as sulphuric acid can be used for pH adjustment, or acidic buffer solutions, such as sodium citrate can be used. In an acidic system, the pH is preferably adjusted to a level of between 2 and 6, and more preferably between the level of 3 and 5. Most preferably, however, the process is conducted under alkali pH conditions.

[0029] The amount of acid or alkali added is dependent upon the original pH of the system and the desired target pH of the process. Also, additional acid or alkali may need to be added to the process during repulping in order to maintain the pH in the desired range.

[0030] The temperature of the repulping process is preferably in the range of from 40° C. to 120° C., more preferably 50° to 90° C., and most preferably 60° C. to 80° C.

[0031] The pulp consistency of the pulp to be treated is preferably in the range of from 1% to 30%, more preferably between 5% to 25%, and most preferably between the levels of 8% to 15%, by weight.

[0032] The wet-strength resins which can be removed by the process of the present invention include polyaminopolyamide-epichlorohydrin resins, urea-formaldehyde and melamie-formaldehyde resins, and the like.

[0033] The surfactant is preferably added to provide a surfactant level of between 0.1 to 10% by weight, and more preferably to provide a surfactant level of between 0.3 to 7.5%. Most preferably, the surfactant level is between 0.5 and 5% by weight.

[0034] The surfactant and the non-chlorinated oxidizer may be premixed and packaged together. Accordingly, in a yet still further aspect, the present invention provides a composition for use in the repulping process of the present invention, which composition comprises a mixture of an non-chlorinated oxidizer and an surfactant.

[0035] The composition is preferably a dry blend composition, and preferably comprises between 0.1 to 20% (by weight of the composition) of surfactant. More preferably, the surfactant level in the composition is between 1 and 15%, and more preferably is between 3 and 10%, by weight.

[0036] It should also be noted that shipment of oxidizers, such as potassium persulfate, can be more difficult than other products because of the oxidizing potential of the persulfate. These shipping restrictions can be reduced if the amount of oxidizer is reduced by the addition of the surfactant. However, it must be emphasized that the surfactant used must be sufficiently compatible with the oxidizer in order to achieve this ability.

[0037] The surfactant (as may also be termed as the emulsifier or the wetting agent component) may be chosen from the wide range of emulsifying agents known in the art. This includes cationic, anionic and nonionic surfactants. Particularly preferred, however, are nonionic surfactants.

[0038] Examples of suitable surfactants include alcohol alkoxylates, phenol alkoxylates, poly(oxyalkylene) glycols, poly(oxyalkylene) fatty acid esters, amine alkoxylates, fatty acid esters of sorbitol and glycerol, fatty acid salts, sorbitan esters, poly(oxyalkylene) sorbitan esters, fatty amine alkoxylates, poly(oxyalkylene)glycol esters, fatty acid amides, fatty acid amide alkoxylates, fatty amine, quaternary amines, alkyloxazolines, alkenyloxazolines, imidazolines, alkyl-sulfonates, alkylarylsulfonates, alkylsulfosuccinates, alkylphosphates, alkenylphosphates, phosphate esters, lecithin, copolymers of poly(oxyalkylene) glycols and poly(12-hydroxystearic acid), condensation products of compounds comprising at least one primary amine and poly[alk(en)yl]succinic acid or anhydride, and mixtures thereof.

[0039] Among the preferred emulsifying agents are alkoxylated alcohols, and in particular ethoxylated C9 to C11 alcohols, 2-alkyl- and 2-alkenyl-4,4′-bis(hydroxymethyl)oxazolines, the fatty acid esters of sorbitol, lecithin, copolymers of poly(oxyalkylene)glycols and poly(12-hydroxystearic acid), condensation products of compounds comprising at least one primary amine and poly[alk(en)yl]succinic acid or anhydride, and mixtures thereof.

[0040] The reaction time will vary depending on the wet strength resin and the alkali charge, but the reaction time can be operable from between 1 minute of contact time to 280 minutes of contact time. Preferably, however, the reaction time should be in the range of 5 to 120 minutes, and most preferably, the reaction time is between 60 and 120 minutes.

[0041] During a typical process of the present invention, the wet-strength resin coated paper broke is first transported to a reactor where an oxidizer salt such as potassium persulfate and an alkali, such as sodium hydroxide, which is used for pH adjustment, are added and intermixed with the paper material with agitation at elevated temperatures.

[0042] In a preferred embodiment, a surfactant is also added to the system. In a most preferred embodiment, a composition comprising a premixed combination of surfactant and persulfate is added.

[0043] The wet-strength coated paper broke, having been processed in the reactor is then conveyed to a screening station. During this conveyance, water is added to the paper slurry in order to provide a screenable feed with a consistency of approximately 3%. The separated fibres pass through the screening means and are recyclable after being washed with an aqueous feed on a cylindrical drum washer. Rejects which do not pass through the screening means and consist of agglomerated fibres and any solid foreign material are removed for disposal or burning. Optionally, the rejects can be recycled to the upstream portion of the reactor for further treatment in an attempt to separate the fibre constituents of the rejects. Dissolved polymer components are removed from the acceptable fibres during the course of the washing cycle. The washed, separated fibres are then in a condition to be utilized for paper production, and they can be recycled to the production process in which they originally formed the broke. In this manner, the fraction of paper production which becomes broke, and which has been known to be as high as 20% of paper production, is effectively reintroduced into the system and fibre losses are therefore greatly minimized.

[0044] The following examples are exemplary of the invention but should not in any way be construed to be a limitation of the process practised and described herein.

EXAMPLES

[0045] Three trials were conducted using i) sodium persulfate only, ii) potassium persulfate only, and iii) a mixture of 90% potassium persulfate in combination with 10% of a nonionic surfactant. The pH of the systems were adjusted to between 10 and 11 before addition of the oxidizer or oxidizer/surfactant mixture. The samples treated were large bales or rolls of paper (generally of over 1 tonne), and the reaction time was typically on the order of 60 minutes.

[0046] To achieve comparable results of resin removal, it was found that the amount of oxidizer needed for experiment 2 or 3 was significantly lower than that required for experiment 1. Usually, the amount of potassium persulfate required was approximately 25% of the weight of the sodium persulfate to achieve the same results. Further, it was found that the results for experiment 3 were achieved more rapidly than for experiment 2. Thus, it was demonstrated that the potassium persulfate system is preferred over a sodium persulfate system, in contrast to the current practise of the industry, and that the speed of repulping could be accelerated by the addition of the surfactant.

[0047] Having described specific embodiments of the present invention, it will be understood that modifications thereof may be suggested to those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claims. Additionally, for clarity and unless otherwise stated, the word “comprise” and variations of the word such as “comprising” and “comprises”, when used in the description and claims of the present specification, is not intended to exclude other additives, components, integers or steps. 

We claim:
 1. A repulping process for use on waste paper containing a wet-strength resin, comprising treating said waste paper, as an aqueous slurry, with a mixture of an alkali hydroxide and essentially 100% potassium persulfate.
 2. A process as claimed in claim 1 wherein said process is conducted as a pH of between 8 to 12, and at a temperature the range of from 40° C. to 120° C.
 3. A process as claimed in claim 2 wherein said temperature range is between 60° C. to 80° C.
 4. A repulping process for use on waste paper containing a wet-strength resin, comprising treating said waste paper, as an aqueous slurry, with a non-chlorinated oxidizer and a surfactant.
 5. A process as claimed in claim 4 wherein said non-chlorinated oxidizer is an alkali or alkali earth metal persulfate or ammonium persulfate.
 6. A process as claimed in claim 5 wherein said oxidizer is an alkali metal persulfate.
 7. A process as claimed in claim 6 wherein said oxidizer is potassium persulfate.
 8. A process as claimed in claim 4 wherein the level of the non-chlorinated oxidizer is between 0.1 to 10% by weight on the weight of pulp and oxidizer.
 9. A process as claimed in claim 8 wherein the level of the oxidizer is between 0.5 to 5% by weight.
 10. A process as claimed in claim 4 wherein said surfactant is a cationic, anionic and nonionic surfactant.
 11. A process as claimed in claim 10 wherein said surfactant is a nonionic surfactant.
 12. A process as claimed in claim 10 wherein said surfactant is an alkoxylated alcohol.
 13. A process as claimed in claim 12 wherein said surfactant is an ethoxylated C9 to C11 alcohol.
 14. A process as claimed in claim 4 wherein the level of said surfactant is between 0.1 to 10% by weight.
 15. A process as claimed in claim 4 wherein the process is conducted at a pH of between 8 to 12; at a temperature in the range of from 40° C. to 120° C.; and at a pulp consistency of between 1% to 30% for a reaction time of from 1 to 280 minutes
 16. A process as claimed in claim 15 wherein the process is conducted at a temperature of between 60° C. to 80° C., for a reaction time of from 60 to 120 minutes.
 17. A composition for use in the repulping of an aqueous slurry of a waste paper containing a wet-strength resin, which composition comprises a mixture of a non-chlorinated oxidizer and an surfactant.
 18. A composition as claimed in claim 17 wherein said composition is a dry blend composition comprising between 0.1 to 20% (by weight of the composition) of said surfactant.
 19. A composition as claimed in claim 17 wherein said composition comprises a mixture of an alkali metal or alkali earth metal persulfate or ammonium persulfate, and a nonionic surfactant.
 20. A composition as claimed in claim 20 wherein said persulfate is potassium persulfate, and said surfactant is an ethoxylated C9 to C11 alcohol. 